Up-to-date fingerprint matching systems using fingerprint image transfer into electronic data usually apply the known contact method to create a fingerprint pattern. A surface topography of a finger is approximated by a series of ridges with intermediate valleys. When a finger is applied to a smooth surface of a transparent optical plate or prism, the ridges contact the optical plate while the valleys do not and instead serve to form the boundaries of regions of air and/or moisture.
The finger to be imaged is illuminated by a light source located below or near to the optical plate. Imaging light from the light source is incident on the smooth surface at an angle of incidence measured with respect to a normal to the smooth surface. Imaging light reflected from the smooth surface is detected by an imaging system that usually includes some form of a detector.
Components of a typical fingerprint imaging system are oriented so that an angle of observation (defined to be an angle between an optical axis of the imaging system and the normal to the smooth surface) is greater than a critical angle for the interface between the smooth surface and the air at the smooth surface. The critical angle at the surface/air interface is defined as the smallest angle of incidence for which imaging light striking the surface/air interface is totally internally reflected within the surface. Therefore, the critical angle at the surface/air interface depends on the index of refraction of the air and optical plate. Another constraint for the angle of observation arises because there is incentive to observe the image at the smallest practical angle of observation, as this reduces distortion due to object tilting. Therefore, the angle of observation is typically chosen to be close to, but greater than the critical angle at the surface/air interface.
At locations where the ridges of the finger contact the smooth surface, total internal reflection does not occur because the index of refraction of a finger is larger than that of air. In this case, imaging light incident on the surface of the optical plate at a location where the ridge of the finger contacts the surface is refracted through the surface/finger interface and then partially absorbed and partially diffused upon contact with the finger. In this case, only a small fraction of incident imaging light is reflected back to a detector of the imaging system.
The imaging system may be implemented to produce bright components at valley locations, thus producing a dark or positive fingerprint pattern. In this case, the imaging system detects the imaging light reflected from the surface/air interface. Alternatively, the imaging system may be implemented to produce bright components at ridge locations, thus producing a bright or positive fingerprint pattern. In this case, the imaging system detects a small percentage of the imaging light that is diffused upon contact with the finger.
Fingerprint imaging systems, in versions that produce positive and negative fingerprint patterns, are, for instance, described in U.S. Pat. No. 4,924,085 to Kato et al.; U.S. Pat. No. 5,596,454 to Hebert, and U.S. Pat. No. 5,796,858 to Zhou, et al. The size of the fingerprint imaging systems described in these patents considerably exceeds a size of the finger receiving surface. Furthermore, the fingerprint imaging systems described in these patents are relatively thick, thus making it difficult to use these devices in a compact device.